Stratified-charge direct injection (SCDI) in a direct injection spark ignition combustion system has been proven to significantly improve engine fuel economy. To produce stratified charge combustion, fuel is directly injected into cylinders of an engine during the compression stroke. This approach enables the mixture to be stratified so that a flammable mixture may be formed in the region of the spark plug while the overall air-fuel ratio is lean.
Bowled piston designs may provide various advantages for stratified-charge operation. Pistons have been designed to expand the region or window of the stratified charge combustion. For example, the U.S. Pat. No. 6,336,437 describes a piston that appears to expand the region of the stratified charge combustion toward the high engine load side. In another example, the U.S. Pat. No. 6,651,612 discloses a piston with a cavity formed in its top face to provide a stratified charge combustion at high engine speed.
However, the inventors have recognized that the problems exist for the stratified charge combustion at lower loads. For example, the engine emissions such as hydrocarbon and NOx emissions may become an issue with a wide stratified-charge operation window. In particular, NOx emissions may not meet the global emission requirement (e.g., European stage V and the U.S. standard) at a light-load stratified-charge operation or at cold start stratified operation.
Thus, according to one aspect, a piston is configured to reciprocate upward and downward within a combustion chamber of a direct injection spark ignition internal combustion engine along a reciprocating axis. The piston comprises an upper end which partly defines the combustion chamber; a bowl defined on the upper end of the piston, the bowl having an at least partially curved sidewall region extending upward from a bottom surface of the bowl, the bowl being configured so that fuel injected laterally into the bowl toward the curved sidewall region from an injection side of the bowl is directed along the curved sidewall region and upward toward a spark plug of the internal combustion engine, where the bowl has a converging contour in which a curvature of the converging contour is greater at a portion of the contour near the spark plug than a portion of the contour near the injection side of the bowl.
The converging bowl contour may help to bring fuel vapor cloud to the spark plug. Since the engine stability is closely related to the local air-fuel mixture distribution around spark plug gap location at the time of ignition, the piston with converging bowl contour may operate stably at a stratified-charge operation with an extended operating window such as at a light-load stratified-charge operation or at cold start stratified operation.
According to another aspect, a piston in a cylinder of a direct injection spark ignition internal combustion engine is configured to move along a longitudinal axis of the cylinder. The piston comprises an upper end which partly defines the combustion chamber; and a bowl defined on the upper end of the piston and having an at least partially curved sidewall region extending upward from a bottom surface of the bowl toward a spark plug side of the bowl, and a sloped bottom rising upward toward an injection side of the bowl, the bowl being configured so that fuel injected into the bowl toward the curved sidewall region from the injection side of the bowl is directed along the curved sidewall region upward toward a spark plug of the internal combustion engine, wherein the bowl has a converging contour defined at least partially by a first circle and a second circle with offset centers, the second circle being closer to the injection side of the bowl and having a larger radius than the first circle. In one embodiment, the bowl has a substantially vertical segment at the spark plug side of the bowl above the at least partially curved sidewall region.
A piston with above features has additional advantages to promote the local air-fuel mixture distribution around spark plug gap. For example, the vertical segment may promote upward motion for fuel-air mixture to reach the spark-plug location. The relative small radius of curved side wall may also enhance a strong upward motion to push the fuel cloud towards the spark plug gap. The upward slope toward the intake port side may provide smooth transition for the fuel-air mixture to focus to the side wall adjacent to the spark plug.
Engine testing data have shown that an engine with a piston having above configuration operates more stable in the stratified charge operation conditions in a wide range of injection timing and spark advance. In addition, the fuel economy at stratified combustion condition may be improved by 1.2% and NOx emissions are reduced by 1%. Furthermore, the engine may have better exhaust recirculation tolerance in some settings.